1. Field of the Invention
The present invention relates to a vertical continuous casting method and a casting mold therefor, and particularly to a continuous casting process for producing high-quality cast materials having few defects such as surface cracks, oscillation marks and the like.
2. Description of the Related Art
A continuous casting method has recently been employed in a steel making step in order to attempt to increase the yield by simplifying the process and decrease the cost by saving energy.
In order to improve productivity, attempts have also been made to decrease the casting time by increasing the casting speed.
Increased casting speed, however, increases the occurrence of breakouts during casting and surface defects such as surface cracks in the cast material.
It is known that longitudinal cracks are caused by nonuniform cooling in a casting mold.
To prevent the occurrence of longitudinal cracks, it is important to prevent formation of non-uniform solidified shell.
Prior art means proposed for preventing the occurrence of non-uniformity of solidified shell include moderation of cooling of a cast material by providing a grooved inner surface of a water-cooling casting mold as disclosed in Japanese Patent Laid-Open No. 51-50819. Japanese Patent Laid-Open No. 59-153550 discloses the moderation of heat transfer by coating a ceramic on the inner surface of a water-cooling copper casting mold.
The method disclosed in Japanese Patent Laid-Open No. 51-50819, however, does not sufficiently moderate cooling and is thus incapable of preventing the occurrence of surface defects. In the method disclosed in Japanese Patent Laid-Open No. 59-153550, separation is formed between a copper plate and the ceramic phase layer due to a difference in thermal expansion between the copper plate and the ceramic, and thus the mold cannot tolerate practical use.
As a result of detailed investigation of the cooling step of a continuous casting method performed by the inventors using a mold powder, it was found that nonuniform solidification of a cast material is mainly produced by the causes described below with reference to FIG. 11. FIG. 11 is a sectional view showing the state wherein molten steel 5 is continuously cast into a mold 1 through an immersion nozzle 4. The casting mold 1 comprises a mold material 2 and a back frame 3. In the mold 1, the poured molten steel 5 first contacts the mold 1 which is cooled with water to form a thin solidified shell 6.
As the temperature decreases, the solidified shell 6 then contracts and separates from the mold 1 to form a gap, i.e., an air gap layer 11, between the mold 1 and the solidified shell 6. The solidified shell 6 maintains its shape by a support roll 12 and is successively drawn downward by a drawing apparatus (not shown) such as a pinch roll. At this time, the mold powder 8 is sprayed on the meniscus 7 in order to prevent sticking between the mold 1 and the solidified shell 6, and improve lubricating properties. The mold powder 8 also functions to absorb and remove the floating nonmetallic inclusion, keep the molten steel 5 hot, and prevent re-oxidation.
The mold powder 8 is caused to flow into the gap between the mold 1 and the solidified shell 6 and is solidified to form a solid mold powder 9 between the mold 1 and the solidified shell 6. The solidifying point of the mold powder 8 is generally about 1000.degree. to 1100.degree. C. On the other hand, the surface temperature of the mold is designed to be 400.degree. C. or lower, since the conventional mold material 2 is generally made of copper alloy which although has high thermal conductivity, suitable for the mold material, high-temperature strength of copper is not so high.
The mold powder 8 is thus solidified on the side of the mold material 2 to form the air gap layer 11 between the mold powder 8 and the casting mold 1. The formation of such an air gap layer 11 significantly decreases the quantity of heat extracted from the mold 1 and thus causes the formation of the nonuniform solidified shell 6. This causes the occurrence of longitudinal cracks on the surface of the cast material and breakouts thereof in combination with thermal stress.